Enthalpy

Enthalpy

energy stored in chemical bonds is released or taken up in chemical reactions.

Endothermic

when heat/energy is added to create a reaction

"Reaction in which energy is absorbed from surroundings Enthalpy c

hange = +ve

Forms products with weaker bonds than the reactants"

Exothermic

when heat/energy is released from a reaction

"Reaction that releases heat to surroundings Enthalpy change = -ve

Forms products with stronger bonds than the reactants"

Activation energy

the amount of energy needed for a reaction to occur.

Bond energy

energy required to break a bond

Delta H

change in enthalpy

kJ/mol

units of enthalpy

Activated complex

the intermediate point where products can start to be formed. Also called transition state.

Standard enthalpy change of reaction

enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions

△H= final - initial

standard conditions: temperature (298K) pressure (100 kPa or 1atm) all substances pure and in their standard stateStandard enthalpy change of reaction

Heat energy change

to calculate this when the temperature of a pure substance is changed, use:

q=mc△t

Exothermic processes

combustion & neutralization

Average bond enthalpy

the energy required to break 1 mole of covalent bonds in the reactant, all reactants and products being in the gaseous state. It is an average value because it takes into account the different energies in a bond between the same atoms in different molecules

standard state

its most stable state under the specified conditions. A precisely defined reference state as:

- Temperature of a substance at exactly 298K

- Pressure of gaseous substance are exactly 1 atm

- liquids & solids are pure

- aqueous solutions are exactly 1 mol per dm cubed

Standard enthalpy change of formation (△Hf)

The standard enthalpy of formation is the energy change when 1 mole of a compound is formed from its constituent elements in their standard states.

Standard Enthalpy Change of Formation (△HC)

The standard enthalpy of combustion is the energy released when 1 mole of a compound burns in excess air or oxygen.

Reaction enthalpy

sum of the products formation enthalpies - the sum of the reactants formation enthalpies.

State and explain the factors that increase the entropy of a system

The number of particles

The nature of the particles

The available energy of the particles

The states ranging from most entropy (most moles) to least entropy (least moles)

Gases→Aqueous→Liquid→Solid

△S= +

increasing chaos

△S= -

decreasing chaos

How to predict whether the entropy change(△S) for a reaction or process is positive or negative

△G= △H- T△S

If theT△S is smaller than △H

△G will be positive and it will be non-spontaneous

→ENDOTHERMIC

If the T△S is larger than △H

△G will be negative and it will be spontaneous

→EXOTHERMIC

Bond enthalpies

bond enthalpies are quoted for the gaseous state and should be recognized as average values obtained from a number of similar compounds